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A mineral consisting of a hydrated phosphate of aluminium and copper, CuAl6(PO4)4(OH)8.4H2O), that is prized as a semiprecious stone. It crystallizes in the triclinic system and is generally blue in colour, the ‘robin's egg’ blue variety being the most sought after. It usually occurs in veinlets and as masses and is formed by the action of surface waters on aluminium-rich rocks. The finest specimens are obtained from Iran.

Subjects: Science and Mathematics — Arts and Humanities.

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